BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.6. An overview of sexual reproduction in vertebrates

Vertebrate evolution reflects a gradual adaptation to terrestrial life. One of the primary hurdles that had to be overcome in transitioning from an aquatic to a terrestrial existence was reproduction.

Fish

Most fish release their Gametes directly into the Water, resulting in external Fertilization. Their eggs contain a relatively large amount of yolk; many species have distinct larval stages, and parental care is rare.

Amphibians

Amphibians must return to water to reproduce, and the Cytology/cytology/16.html">Early stages of their development also take place in an aquatic environment. However, many amphibian species exhibit remarkably sophisticated forms of parental behavior. For instance, the male Surinam toad distributes fertilized eggs across the female's back, where they adhere, as if "sinking" into the Skin. The eggs develop right there, eventually hatching into tadpoles. In about three weeks, the tadpoles leave the mother's back to begin an independent life.

Reptiles

Reptiles were the first vertebrates to solve Structure/149.html">The problem of fertilization and development on land. Gametes deposited on dry ground would be doomed to desiccate; therefore, the primary prerequisite for a complete transition to terrestrial existence was the Introduction of male gametes into the female's body, i.e., internal fertilization. This is precisely how fertilization occurs in reptiles, and because this increases the likelihood of gamete fusion, the total number of gametes produced can be reduced. The resulting zygote develops within a specialized structure known as the amniotic (or cleidoic) egg. This egg provides the embryo with a fluid-filled cavity in which development can safely proceed (Fig. 21.31). Externally, the egg is encased in a protective shell that guards against mechanical damage. Beneath this outer shell lie four extraembryonic membranes derived from embryonic Tissues. These membranes—the yolk sac, amnion, chorion, and allantois—protect the embryo and support numerous metabolic Functions, notably Nutrition, Respiration, and excretion.

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Fig. 21.31. Simplified diagram of The structure of an amniotic egg.

The yolk sac originates as an outgrowth of the embryonic gut; it envelops the yolk—a nutritional reserve that is gradually absorbed by vitelline Blood Vessels. Once the yolk is entirely depleted, the yolk sac shrinks and is drawn back into the gut.

The amnion completely encloses the embryo within the amniotic cavity. Amniotic Cells secrete the Amniotic Fluid that fills this cavity, thereby immersing the embryo in a liquid medium essential for its development. The amnion is present in all reptiles, birds, and mammals, which is why these groups are collectively termed amniotes. As the embryo grows, the amnion is pushed outward until it fuses with the third extraembryonic membrane, the chorion, which lies directly beneath the shell and prevents excessive water loss from the amnion.

The fourth membrane, the allantois, forms as an outgrowth of the embryonic hindgut. In reptiles and birds, it expands rapidly to form a lining just beneath the chorion. It functions primarily as a "Urinary Bladder" for the accumulation of Metabolic waste products; additionally, it facilitates gas exchange between the external environment and the amniotic fluid through the porous eggshell.

Birds and egg-laying mammals

Birds and early egg-laying mammals, known as monotremes—such as the platypus (Ornithorhyncus)—lay amniotic eggs. The eggs of higher mammals have lost the protective shell but retained the four embryonic membranes; in placental mammals, two of these membranes, the chorion and the allantois, combine to form the Placenta (Fig. 21.49).